Antibodies against primary structure

What is the minimum number of amino acid residues that an antibody can
detect? We are trying to model the loss of immunological epitopes in
fossil proteins as a function of chain scission, the rate of which is
being monitored as the release of free amino acids. We are using
polyclonal antisera, but assume that the most persistent epitopes are to
primary structure and these are the ones that are capable of detecting
samples which are > 1 million years old.
To get the model to fit our data requires a minimum length of intact
sequence that an antibody can detect of between three and four residues
but this is much shorter than the 6-7 reside minimum I was taught as an
undergraduate, implying that our model is wrong.
Does anyone know the answer (preferably with a citable source)?